1 Effects of Film Forming Compounds on the growth of Giant CCN: Implications for cloud microphysics and the aerosol indirect effect.

Abstract. The presence of Giant cloud condensation nuclei (GCCN) within stratocumulus clouds can help the formation of drizzle by acting as collector drops. We propose that the presence of Film Forming Compounds (FFCs) on GCCN may decrease their growth enough to cease this drizzle formation mechanism. We systematically explore the accommodation properties and amount of FFCs necessary to have a significant impact on GCCN size under realistic conditions of growth inside typical stratocumulus clouds. It is found that even low mass fractions (as low as 0.2%) of FFCs with a modest effect on water vapor accommodation can significantly reduce GCCN size and their potential to act as collector drops. Our conclusions apply to both pristine and polluted aerosol conditions, which suggest that in the presence of FFCs, GCCN may be influencing the microphysical evolution of clouds to a lesser extent than previously thought.